Fuel cells are useful sources of clean electricity. One type of common fuel cell is fuelled by hydrogen and oxygen or air (which contains about 19 percent oxygen) to generate direct current electricity. Such fuel cells are clean, highly efficient and environmentally attractive.
It is useful to be able to monitor the performance of hydrogen-oxygen or hydrogen-air fuel cells to ensure that they are operating efficiently. It is also important to be able to detect any deterioration in performance before the fuel cell becomes a hazard to equipment investment and human beings. A hydrogen-oxygen fuel cell usually develops about 0.6 to 1.0 volt of electricity. In order to generate higher voltages, and more power, it is common practice to arrange a number of cells electrically in series in what is referred to as a fuel cell stack. As a stack may contain 50 or more cells, it is difficult and prohibitively expensive to monitor the performance of each individual cell in a stack.
It is important, however, to assure that the power being drawn from the stack does not force any individual cell into an abusive or hazardous operating range. It is also important to be able to detect the failure of fuel/oxidant separation in a single cell. It is desirable to monitor cell performance as part of a system control strategy for the equipment which supports the operation of the fuel cell.
Sufficient information cannot be obtained from the total stack voltage, particularly in circumstances of significantly varying load, as the change in the stack voltage can be large as compared to the voltage of a single cell. It is also not practical to store in a control system a table of acceptable stack voltages over the operative power range, as these values will vary with operating temperature, fuel and oxidant pressure, outside air pressure, temperature and humidity, and the extent of stack aging.
It is desirable, therefore, to be able to monitor the cell voltages with sufficient resolution to detect inadequate performance in a single cell and to have a reference voltage which reflects the expected value for the given operating conditions and stack age.
The inventors are not aware of such an approach as described herein having been used in monitoring and controlling fuel cells. The only remotely analogous prior art known to the inventors is the monitoring of the voltages of individual batteries in installations of many batteries connected in series.
As used herein, the term "normalized measured voltage" refers to the measured voltage across a group of fuel cells, normalized according to the number of fuel cells in the group. Similarly, the term "normalized total voltage" refers to the measured voltage across the total number of fuel cells, or alternately the cumulative total of the measured voltages across each fuel cell group, normalized according to the total number of fuel cells. For example, if twenty total fuel cells are divided into four groups composed of (a) three, (b) four, (c) six (d) and seven cells, respectively, and the measured voltages across the groups is (a) 1.2 volts, (b) 2.0 volts, (c) 3.0 volts, and (d) 4.2 volts, then the normalized measured voltage is 0.4 volts per cell for group (a), 0.5 volts per cell for group (b), 0.5 volts per cell for group (c) and 0.6 volts per cell for group (d). The normalized total voltage in the above example is 1.2+2.0+3.0+4.2 volts divided by 20 cells, or 0.52 volts per cell. Use of normalized voltages permits the comparison of measured voltages between groups composed of unequal numbers of fuel cells. In practice, the comparison of performance indicators between groups can be made on a per cell basis or on a per group basis where the groups consist of equal numbers of cells.